Malaria ranks as one of the world's top three deadliest diseases (approximately 300 million cases per year). P. falciparum causes the most severe form, causing the death of about 1 million people annually (90 percent of whom are young children).
Infection begins when malaria sporozoites are injected into the bloodstream of a host by a mosquito. After injection, they migrate to the liver and multiply in hepatocytes for one to two weeks. The sporozoites differentiate to merozoites which are released from the liver into the blood stream, where they infect erythrocytes. When the merozoite matures in the red blood cell, it is known as a trophozoite and, when fully developed, as a schizont. A schizont is the stage when nuclear division occurs to form individual merozoites which are released to invade other red cells. After several schizogonic cycles, some parasites, instead of becoming schizonts through asexual reproduction, develop into large uninucleate parasites. These parasites undergo sexual development.
Sexual development of the malaria parasites involves the female macrogametocyte and the male microgametocyte. If a mosquito feeds on the blood of an infected host, it can ingest gametocytes within the blood. Fertilization and sexual recombination of the parasite occurs in the mosquito's gut. The fertilized parasite, which is known as a zygote, then develops into an ookinete. The ookinete penetrates the midgut wall of the mosquito and develops into an oocyst, within which many small sporozoites form. When the oocyst ruptures, the sporozoites migrate to the salivary gland of the mosquito via the hemolymph. Once in the saliva of the mosquito, the parasite can be injected into a host, repeating the life cycle.
There is no vaccine, and malarial parasites are increasingly becoming resistant to antimalarial drugs that have been used to treat the disease for decades.